Your lawn should not look like this now. If your lawn has brown spots like these, they may be from sod webworms. Photo by David W. Marshall.
Sod Webworms Attacking Lawns
Guest Article for the Tallahassee Democrat
October 16, 2015 Release for Tallahassee Democrat
By: David W. Marshall
I just picked up a bag of winterizer lawn fertilizer (5-0-20) containing bifenthrin, an insecticide, from my local independent garden center. My objective is to kill the tropical sod webworms, which are eating my lawn, and give the lawn a boost before the cold weather arrives. Otherwise, I’m afraid my lawn will be in very bad shape come next spring.
If your lawn has had brown areas develop in it over the last month or so, you may want to take similar action. Your lawn should not be turning brown yet. It’s not the fall weather. It’s most likely the sod webworms.
How do you know for sure? Take a close look at the damaged areas, but where there’s still a little green grass left. Do you see notched areas on the edges of some of the green leaf blades, indicating chewing damage? If you do, then you probably have sod webworms. Don’t be surprised that you don’t see the caterpillars themselves. They feed at night, and during the day they will be inconspicuously curled up near the soil line and you probably won’t see them. You may see the small grayish adult moths flitting around the lawn and the perimeters. But they aren’t the stage that feeds on the grass. So don’t be overly concerned with them, other than knowing that if cold weather doesn’t arrive until late fall, the moths can continue to lay eggs and you may need a second insecticide application later.
If you have sod webworm damage, it’s not wise to just let it go, thinking that the lawn will be going dormant soon anyway. You want the lawn to go into winter in good shape. Otherwise, don’t expect it to come out in good shape in the spring. It will be much more costly to have to re-sod next spring than to try and stop the webworm damage now and get some new growth on the grass before cold weather arrives.
That’s why I chose the 5-0-20 fertilizer containing bifenthrin. The fertilizer is low in nitrogen and high in potassium. It will give the lawn a little push now but it doesn’t contain so much nitrogen that the lawn will still be growing too actively when cold weather does arrive. And the high potassium to nitrogen ratio will increase the lawn’s hardiness. The bifenthrin is to control the sod webworms.
There are also other lawn insecticides that can be used. Just be sure to choose one that has directions for sod webworms on the label. And be sure to follow the label directions carefully. If you want to make sure that you don’t harm any other type of insects, select a lawn insecticide containing Bacillus thuringiensis. It only kills caterpillars. But the product breaks down quickly in sunlight, so again, be sure to follow label directions carefully and apply it only in late afternoon.
Lawn winterizer fertilizers can also be purchased without insecticide. Just be sure to select one in a low nitrogen to high potassium ratio such as 5-0-20 or 5-0-15. And make sure that you lightly water in the fertilizer after application.
Keep an eye on your lawn even after you apply an insecticide and fertilize. You want to ascertain that new damage doesn’t occur and that your lawn starts to recover. If the weather turns dry, you may need to water. Keep the falling leaves from accumulating on the lawn too. You may be tired of lawn care for the year. But taking these steps now can help you to have a better lawn next spring when you want it.
David W. Marshall is landscape consultant with Esposito Garden Center and author of Design & Care of Landscapes & Gardens in the South. David is also Extension Agent Emeritus and a volunteer writer for Leon County UF/IFAS Extension. For gardening questions, email us at Ask-A-Mastergardener@leoncountyfl.gov